This PR fixes a panic when compiling mutually recursive definitions that
use `casesOn` on indexed inductive types (e.g. `Vect`). The
`splitMatchOrCasesOn` function in `WF.Unfold` asserted
`matcherInfo.numDiscrs = 1`, but for indexed types the casesOn recursor
has multiple discriminants (indices + major premise). The fix uses the
last discriminant (the major premise) and lets the `cases` tactic handle
index discriminants automatically.
Closes#13015
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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